Display device and electronic device

ABSTRACT

A display device has an NTSC ratio of higher than or equal to 80% and a contrast ratio of higher than or equal to 500 and includes a display portion. In the display portion, a pixel is provided at a resolution of greater than or equal to 80 ppi, and the pixel includes a light-emitting module capable of emitting light with a spectral line half-width of less than or equal to 60 nm. Further, the light emission of the light-emitting module is raised to a desired luminance with a gradient of greater than or equal to 0 in response to an input signal within a response time of longer than or equal to 1 μs and shorter than 1 ms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/875,588, filed May 2, 2013, now allowed, which claims the benefit ofa foreign priority application filed in Japan as Ser. No. 2012-107944 onMay 9, 2012, both of which are incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a display device. Further, the presentinvention relates to an electronic device including a display device.

2. Description of the Related Art

A display device that displays a stereoscopic image using a binocularparallax is known. Such a display device is configured to display, onone screen, an image to be seen from the position of the left eye of aviewer (an image for left eye) and an image to be seen from the positionof the right eye of the viewer (an image for right eye). The viewer seesthe image for left eye with the left eye and the image for right eyewith the right eye and is thus allowed to see a stereoscopic image.

For example, in a glasses system, an image for left eye and an image forright eye are alternately displayed on a screen of a display device insynchronization with a shutter provided in a pair of glasses, wherebythe left eye of a viewer is allowed to see only the image for left eyeand the right eye of the viewer is allowed to see only the image forright eye. Thus, the viewer can see a stereoscopic image.

Further, in a display device using a parallax barrier system whichallows a viewer to see a stereoscopic image with naked eyes, a screen isdivided into many regions (e.g., strip-like regions). These regions arealternately allocated to right eye and left eye, and a parallax barrieris provided to overlap with the boundaries of the regions. In therespective divided regions, an image for right eye and an image for lefteye are displayed. With the parallax barrier, the regions for displayingthe image for right eye are hidden from the left eye of a viewer and theregions for displaying the image for left eye are hidden from the righteye of the viewer; consequently, the left eye is allowed to see only theimage for left eye and the right eye is allowed to see only the imagefor right eye. Thus, the viewer can see a stereoscopic image.

Note that a display device including a switchable parallax barrier forachieving switching between a flat image display mode and a stereoscopicimage display mode is known (Patent Document 1).

In addition, a light-emitting element in which a layer containing alight-emitting organic compound is provided between a pair of electrodesis known. This light-emitting element is a self-luminous type;therefore, high contrast and high speed of response to an input signalare achieved. A display device in which this light-emitting element isused is known (Patent Document 2).

REFERENCE Patent Document

[Patent Document 1] International Publication WO2004/003630 Pamphlet

[Patent Document 2] Japanese Published Patent Application No.2011-238908

SUMMARY OF THE INVENTION

In the case of using a display device that displays a stereoscopic imageusing a binocular parallax, a distance between a screen of the displaydevice and the left eye or the right eye of a viewer is almost uniformregardless of an image displayed. Therefore, in some cases, a distancebetween the viewer and a screen on which the right eye or the left eyeof the viewer is focused is different from a distance, which provides abinocular parallax, between the viewer and the object in an imagedisplayed on the screen. Thus, there has been a problem in that thedifference has caused strain on the viewer.

The present invention is made in view of the foregoing technicalbackground. Therefore, an object of one embodiment of the presentinvention is to provide a display device that can display an image whichcauses a viewer less stain associated with viewing and gives a viewer agreat sense of depth. Further, an object of one embodiment of thepresent invention is to provide an electronic device for enjoying animage which causes a viewer less strain associated with viewing andgives a viewer a great sense of depth.

In order to achieve at least one of the objects, one embodiment of thepresent invention is made by focusing on response characteristics of adisplay device with respect to an image signal. Specifically, as for adisplay element provided in a pixel portion of the display device, thepresent inventors have focused the fact that transient characteristicsuntil the completion of response to an input signal deeply influence onstereoscopic viewing. This leads to a display device having a structureexemplified in this specification and an electronic device using thesame.

That is, a display device according to one embodiment of the presentinvention has an NTSC ratio of higher than or equal to 80% and acontrast ratio of higher than or equal to 500 and includes a displayportion. In the display portion, a pixel is provided at a resolution ofgreater than or equal to 80 ppi, and the pixel includes a light-emittingmodule capable of emitting light with a spectral line half-width of lessthan or equal to 60 nm. Further, the light emission of thelight-emitting module is raised to a desired luminance with a gradientof greater than or equal to 0 in response to an input signal in aresponse time of longer than or equal to 1 μs and shorter than 1 ms.

Since most subjects in nature have a curved surface, when the viewersees that reflected light from the surface of the subject changesdepending on changes in relative positions among the subject, a lightsource, and a viewer, the luminance seen by the eyes of the viewerchanges with a gradient with respect to time.

Therefore, when the viewer sees light emission having such transientcharacteristics in which the rise of the emission intensity in responseto an input signal is gradual, stimulation on the brain of the viewer atthe instant of switching of continuous images is relieved. Thus, it ispossible to display an image that is seen as light faithful to thereflected light in nature without strain and that can give a naturalsense of depth.

Here, when the response time is longer than or equal to 1 ms forexample, afterimages of moving images in which a subject moves largelymay be seen, failing to give reality and a natural sense of depth. Onthe other hand, with light emission with an extremely fast response timewhich is shorter than 1 μs, it is difficult to obtain light emissionfaithful to reflected light in nature.

Further, in the display portion included in such a display device, thedistribution of light and shade in an image can be widened and thus adetailed image can be displayed. Furthermore, an image which is faithfulto a camera-captured image can be displayed smoothly. Accordingly, aviewer is given a greater sense of depth by monocular vision, which caneliminate the need for displaying images including a binocular parallaxon one screen. In addition, a viewer can see an image with naked eyes.As a result, it is possible to reduce strain on a viewer which is causedby viewing and to display an image which allows a viewer to have a greatsense of depth.

In addition, a pixel includes light-emitting modules each emitting lightwith a narrow spectral line half-width and high color purity, whichincreases NTSC ratio and contrast. Thus, an image with a wide grayscalerange can be displayed. Since the pixel includes a light-emittingelement having a short response time, an image in motion can bedisplayed smoothly. Thus, a moving image in which a front image movessmoothly and faster than a back image while overlapping with the backimage can be expressed. The wide grayscale range and the smooth motioninteract with each other, which allows a viewer to see an image with agreat sense of depth.

Therefore, a display device having the above structure can provide animage which gives a viewer a great sense of depth without strain.

Further, a display device according to another embodiment of the presentinvention has an NTSC ratio of higher than or equal to 80% and acontrast ratio of higher than or equal to 500 and includes a displayportion and a correction control circuit. In the display portion, apixel is provided at a resolution of greater than or equal to 80 ppi,and the pixel includes a light-emitting module capable of emitting lightwith a spectral line half-width of less than or equal to 60 nm. Further,the correction control circuit is configured to generate a signal thatcorrects a response time of the light emission of the light-emittingmodule and to output the signal to the display portion. Furthermore, thelight emission of the light-emitting module is raised to a desiredluminance with a gradient of greater than or equal to 0 in response toan input signal in a response time of longer than or equal to 1 μs andshorter than 1 ms.

Such a correction control circuit can control transient characteristicsof the emission intensity of light-emitting modules. Examples of asignal generated by the correction control circuit may include acorrection voltage signal that corresponds to a high voltage applied toan element right after an emission start, a correction control signalthat regulates a period during which the voltage is applied to theelement, and the like.

Further, in any of the above display devices according to embodiments ofthe present invention, the light-emitting module preferably includes areflective film, a semi-transmissive and semi-reflective film, and alight-emitting element between the reflective film and thesemi-transmissive and semi-reflective film. Further, the light-emittingelement includes a pair of electrodes and a layer containing alight-emitting organic compound between the pair of electrodes.

With such a structure, as an effect of a micro resonator (also referredto as a microcavity), interference of light occurs between thereflective film and the semi-transmissive and semi-reflective film, andspecific light among light with wavelengths in the visible light regionis strengthened. Accordingly, high color saturation images can bedisplayed by light with a narrow spectral line width (specifically, aspectral line half-width of 60 nm or less), thereby giving a viewer agreater sense of depth. Consequently, it is possible to provide adisplay device that can display an image which causes a viewer lessstrain associated with viewing and gives a viewer a great sense ofdepth.

In any of the display devices according to embodiments of the presentinvention, it is preferable that the light-emitting module include thefollowing: a reflective film; a semi-transmissive and semi-reflectivefilm; a light-emitting element provided between the reflective film andthe semi-transmissive and semi-reflective film, and including a pair ofelectrodes, a plurality of layers containing light-emitting organiccompounds between the pair of electrodes, and an interlayer between thelayers containing light-emitting organic compounds; and a color filteroverlapping with the light-emitting element with the semi-transmissiveand semi-reflective film provided therebetween.

With such a structure, interference of light occurs between thereflective film and the semi-transmissive and semi-reflective film,specific light among light with wavelengths in the visible light regionis strengthened, and unnecessary light is absorbed by the color filter.Accordingly, high color saturation images can be displayed by light witha narrower spectral line width (specifically, a spectral line half-widthof 60 nm or less), thereby giving a viewer a greater sense of depth.Consequently, it is possible to provide a display device that candisplay images which cause a viewer less strain associated with viewingand give a viewer a great sense of depth.

In any of the display devices according to embodiments of the presentinvention, it is preferable that the light-emitting module provided ineach of the pixels be any one of the following: a first light-emittingmodule including a color filter transmitting red light and a reflectivefilm and a semi-transmissive and semi-reflective film between which anoptical path length is adjusted to i/2 times (i is a natural number) alength greater than or equal to 600 nm and less than 800 nm; a secondlight-emitting module including a color filter transmitting green lightand a reflective film and a semi-transmissive and semi-reflective filmbetween which an optical path length is adjusted to j/2 times is anatural number) a length greater than or equal to 500 nm and less than600 nm; and a third light-emitting module including a color filtertransmitting blue light and a reflective film and a semi-transmissiveand semi-reflective film between which an optical path length isadjusted to k/2 times (k is a natural number) a length greater than orequal to 400 nm and less than 500 nm.

Further, in any of the display devices according to embodiments of thepresent invention, it is preferable that the light-emitting moduleprovided in each of the pixels be any of the following: a firstlight-emitting module including a color filter transmitting red lightand a reflective film and a semi-transmissive and semi-reflective filmbetween which an optical path length is adjusted to i/2 times (1 is anatural number) a length greater than or equal to 600 nm and less than800 nm; a second light-emitting module including a color filtertransmitting green light and a reflective film and a semi-transmissiveand semi-reflective film between which an optical path length isadjusted to j/2 times (j is a natural number) a length greater than orequal to 500 nm and less than 600 nm; and a third light-emitting moduleincluding a color filter transmitting blue light and a reflective filmand a semi-transmissive and semi-reflective film between which anoptical path length is adjusted to k/2 times (k is a natural number) alength greater than or equal to 400 nm and less than 500 nm. It is alsopreferable that the first light-emitting module, the secondlight-emitting module, and the third light-emitting module include alayer containing the same light-emitting organic compound.

In any of the above-described display devices according to embodimentsof the present invention, it is preferable that the light-emittingmodule includes a light-emitting element in which one of the pair ofelectrodes also serve as a reflective film and the other also serve as asemi-transmissive and semi-reflective film.

With such a structure, color purity of light emitted from each of thelight-emitting module can be increased. Further, the layers containing alight-emitting organic compound can be formed in the same step.Furthermore, the pair of electrodes can also serve as the reflectivefilm and the semi-transmissive and semi-reflective film. Therefore, amanufacturing process can be simplified. Thus, it is possible to providea display device that is easily manufactured and that can display animage which causes a viewer less strain associated with viewing andgives a viewer a great sense of depth.

In particular, a microcavity is highly effective in narrowing thespectral line half-width and in making a pixel more unnoticeable as theresolution becomes higher. Further, it is easy for a human brain torecognize an image in motion and an image which changes from a stillimage to a moving image Therefore, by increasing color purity and makinga pixel more unnoticeable, a smoother moving image can be displayed;thus, it is possible to provide a display device that can display animage which causes a viewer less strain associated with viewing andgives a viewer a great sense of depth.

In any of the display devices according to embodiments of the presentinvention, it is preferable that the light-emitting module provided ineach of the pixels emit any of the following light: red light with aspectral line half-width of less than 50 nm; green light with a spectralline half-width which is narrower than that of the red light; and bluelight with a spectral line half-width which is narrower than that of thegreen light.

In such a structure, the half-width of the green light whose luminosityfactor is higher than that of the red light is narrower than thehalf-width of the red light, and the half-width of the blue light isnarrower than the half-width of green light. Thus, an image with highsaturation can be displayed with the use of light with a narrow spectralline width (specifically, a spectral line half-width of 50 nm or less),and a depth effect is enhanced.

Another embodiment of the present invention is an electronic deviceincluding any of the above display devices.

According to one embodiment of the present invention, an image with awide distribution of light and shade is displayed on the electronicdevice. Further, an image which is faithful to a camera-captured imagecan be displayed smoothly on the electronic device. Furthermore, a richimage which is faithful to reflected light in nature is displayed on theelectronic device. Accordingly, a viewer is given a greater sense ofdepth by monocular vision, which can eliminate the need for displayingimages including a binocular parallax on one screen. In addition, aviewer can see an image with naked eyes. Thus, it is possible to providean electronic device for enjoying images which cause a viewer lessstrain associated with viewing and give a viewer a great sense of depth.

Note that “optical path length” in this specification means the productof distance and refractive index. Therefore, the optical path length ofa medium having a refractive index of more than 1 is longer than theactual distance. Note that the optical path length in a resonator of amicro resonator can be obtained by measuring optical interference.Specifically, the optical path length in a resonator can be obtained bymeasuring an intensity ratio of reflected light to incident light with aspectrophotometer and then plotting the measured intensity ratio withrespect to a wavelength.

Note that in this specification, the display device includes any of thefollowing modules in its category: a module in which a connector such asa flexible printed circuit (FPC) or a tape carrier package (TCP) isattached to a display panel; a module having a TCP provided with aprinted wiring board at the end thereof; and a module having anintegrated circuit (IC) directly mounted over a substrate, over which adisplay portion is formed, by a chip on glass (COG) method.

According to one embodiment of the present invention, it is possible toprovide a display device that can display an image which causes a viewerless strain associated with viewing and gives a viewer a great sense ofdepth. Further, it is possible to provide an electronic device forenjoying an image which causes a viewer less strain associated withviewing and gives a viewer a great sense of great depth.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B show a structural example of a light-emitting moduleaccording to one embodiment and transient characteristics of emissionintensity;

FIGS. 2A to 2D show structural examples of light-emitting modulesaccording to embodiments and transient characteristics of emissionintensity;

FIGS. 3A and 3B show transient characteristics of the emission intensityof light-emitting modules according to embodiments;

FIG. 4 shows a configuration example of a display device according toone embodiment;

FIG. 5 shows a configuration example of a display device according toone embodiment;

FIG. 6 is a timing chart showing an operation example of a displaydevice according to one embodiment;

FIGS. 7A to 7C show a structural example of a display device accordingto one embodiment;

FIGS. 8A and 8B show a structural example of a display device accordingto one embodiment;

FIGS. 9A and 9B show a structural example of a display device accordingto one embodiment;

FIGS. 10A to 10C each show a structural example of a light-emittingelement according to one embodiment; and

FIGS. 11A to 11E each show a structural example of an electronic deviceincluding a display device according to one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the invention is not limited to the following description, andit will be easily understood by those skilled in the art that variouschanges and modifications can be made without departing from the spiritand scope of the invention. Therefore, the invention should not beconstrued as being limited to the description in the followingembodiments. Note that in the structures of the invention describedbelow, the same portions or portions having similar functions aredenoted by the same reference numerals in different drawings, anddescription of such portions is not repeated.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Embodiment 1

In this embodiment, a structural example of a display device accordingto one embodiment of the present invention is described with referenceto drawings.

Structural Example

FIG. 1A schematically shows a structural example of a light-emittingmodule that can be used for a pixel portion of a display deviceaccording to one embodiment of the present invention.

In a pixel 402, a light-emitting module 450 is provided. Thelight-emitting module 450 includes a first substrate 410, a secondsubstrate 440, and a light-emitting element 420 sealed between the firstsubstrate 410 and the second substrate 440 which are bonded to eachother with an unshown sealant. The light-emitting element 420 includes afirst electrode 421 formed over the first substrate 410, a secondelectrode 422 overlapping with the first electrode 421, and a layer 423containing a light-emitting organic compound between the first electrode421 and the second electrode 422.

The first electrode 421 shown as an example in this embodiment includesa reflective film and a conductive film having a light-transmittingproperty. The conductive film having a light-transmitting property isstacked over the reflective film to be close to the second electrode 422side. Further, the second electrode 422 has a semi-transmissive andsemi-reflective property with respect to visible light. Thus, the firstelectrode 421 and the second electrode 422 form a micro resonator (alsoreferred to as a microcavity).

From the light-emitting element 420 including the micro resonator, lightwith wavelengths in accordance with the distance between the reflectivefilm and the semi-transmissive and semi-reflective film can beeffectively extracted. Specifically, in order to extract light with aspecific wavelength λ, thicknesses of the layer 423 containing alight-emitting organic compound and of the conductive film having alight-transmitting property, which is included in the first electrode421, are preferably adjusted such that an optical path length (theproduct of distance and refractive index) is n times as large as λ/2 (nis a natural number).

Voltage application between the first electrode 421 and the secondelectrode 422 allows the light-emitting module 450 to emit light L tothe outside. The light L has a narrow spectral line half-width in whichlight with a specific wavelength is effectively extracted.

Referring to FIG. 1B, the following shows relations between voltagewaveforms of signals input to the light-emitting module 450 andtransient characteristics of the emission intensity of thelight-emitting module 450. An upper row in FIG. 1B shows transientcharacteristics of the emission intensity of the light-emitting module450, and the second row, the third row, and the fourth row in FIG. 1Bshow voltage waveforms of signals input to the light-emitting module450.

When a signal S0 with a rectangular wave with a rise at a time T0, asshown in the second row in FIG. 1B, is input to the light-emittingmodule 450, emission intensity L0 of the light-emitting module 450 has asharp gradient in a rise portion and reaches a desired luminance (100%).

Here, when a signal S1 with a voltage waveform in which the voltagebecomes a voltage V1 which is lower than a voltage V0 corresponding tothe desired luminance in a period (T0 to T1) right after an emissionstart, as shown in the third row in FIG. 1B, is input to thelight-emitting module 450, emission intensity L1 of the light-emittingmodule 450 can be raised gradually in two steps; thus, the response time(time until when the emission intensity reaches 90% of the desiredluminance) can be significantly delayed as compared to the emissionintensity L0 when the above-described rectangular wave is used.

Since most subjects in nature have a curved surface, when the viewersees that reflected light from the surface of the subject changesdepending on changes in relative positions among the subject, a lightsource, and a viewer, the luminance seen by the eyes of the viewerchanges with a gradient with respect to time.

Therefore, when the viewer sees, as light emission from thelight-emitting module included in the display device, light emissionhaving transient characteristics in which the rise of the emissionintensity in response to an input signal is gradual, stimulation on thebrain of the viewer at the instant of switching of continuous images isrelieved. Thus, it is possible to display an image that is seen as lightfaithful to the reflected light in nature without strain and that cangive a natural sense of depth.

Here, when the response time is longer than or equal to 1 ms forexample, afterimages of moving images in which a subject moves largelymay be seen, failing to give reality and a natural sense of depth. Onthe other hand, with light emission with an extremely fast response timewhich is shorter than 1 μs, it is difficult to obtain light emissionfaithful to reflected light in nature. Therefore, it is preferable thatthe response time of the emission intensity be set to longer than orequal to 1 μs and shorter than 1 ms, and that the voltage waveform of aninput signal be adjusted such that the emission intensity is raised tothe desired luminance with a gradient of greater than 0 in response toan input signal.

Further, such a light-emitting module can be provided in a pixel, andsuch a pixel can be provided at a resolution of greater than or equal to80 ppi, preferably greater than or equal to 300 ppi, in a displayportion of a display device with an NTSC ratio of higher than or equalto 80%, preferably higher than or equal to 95%, and a contrast ratio ofhigher than or equal to 500, preferably higher than or equal to 2000.Thus, it is possible to provide a display device that can display animage which causes a viewer less strain associated with viewing andgives a viewer a great sense of depth. Further, interference of lightemitted from the light-emitting element occurs between the reflectivefilm and the semi-transmissive and semi-reflective film, and specificlight is strengthened. Thus, an image with high saturation can bedisplayed by light with a narrow spectral line width, and a depth effectis enhanced.

Therefore, a display device having the above structure can provide animage which gives a viewer a great sense of depth without strain.

Here, as a signal input to the light-emitting module 450, it is possibleto use a signal S2 shown in the fourth row in FIG. 1B. The signal S2 hasa voltage waveform in which the voltage is raised in a stepwise mannerto the voltage V0 corresponding to the desired luminance in a periodright after the emission start (T0 to T2). In this case, as shown in thefirst row in FIG. 1B, luminance intensity L2 of the light-emittingmodule 450 has transient characteristics in which the emission intensityis raised with an almost constant gradient.

By using the signal having a step-like voltage waveform in this manner,it is possible to control the emission intensity of the light-emittingmodule 450 so as to be raised gradually to the desired luminance and toobtain more natural light emission. Further, by adjusting the voltagewaveform, it is possible to freely set the gradient of the rise of theemission intensity.

Note that the above description is made on the signal having a step-likevoltage waveform; however, a signal having a voltage waveform in whichthe voltage is raised with a gradient in order to make a gradual rise ofthe emission intensity of the light-emitting module 450 may be used.

For simplicity, the following shows a case of using a signal having avoltage waveform in which the voltage becomes a voltage V1 which islower than the voltage V0 corresponding to the desired luminance in aperiod right after the emission start (T0 to T1) as shown in the thirdrow in FIG. 1B.

The above is the description of this structural example.

Light-Emitting Module

The following shows a more specific structural example of alight-emitting module that can be used for a pixel portion of a displaydevice according to one embodiment of the present invention.

Light-Emitting Module 450X

In a pixel 402X shown in FIG. 2A, a light-emitting module 450X isprovided. The light-emitting module 450X includes the first substrate410, the second substrate 440, and a light-emitting element 420X sealedbetween the first substrate 410 and the second substrate 440 which arebonded to each other with an unshown sealant. The light-emitting element420X includes a first electrode 421X formed over the first substrate410, the second electrode 422 overlapping with the first electrode 421X,and a layer 423X containing a light-emitting organic compound betweenthe first electrode 421X and the second electrode 422.

The layer 423X containing a light-emitting organic compound contains afluorescent organic compound, and the light-emitting module 450X emitslight X to the outside. The light X has a narrow spectral linehalf-width in which light with a specific wavelength is effectivelyextracted from light emitted from the fluorescent organic compound.

The light-emitting module 450X contains a fluorescent organic compoundin the layer 423X containing a light-emitting organic compound. Sinceexcitation lifetime of an excited species of the fluorescent organiccompound is relatively short, a response time to an input signal isshort.

Light-Emitting Module 450Y

In a pixel 402Y shown in FIG. 2B, a light-emitting module 450Y isprovided. The light-emitting module 450Y includes a light-emittingelement 420Y instead of the light-emitting element 420X in thelight-emitting module 450X. Further, the light-emitting element 420Yincludes a first electrode 421Y and a layer 423Y containing alight-emitting organic compound instead of the first electrode 421X andthe layer 423X containing a light-emitting organic compound in thelight-emitting element 420X.

The thickness of a conductive film having a light-transmitting propertyincluded in the first electrode 421Y is greater than that of theconductive film having a light-transmitting property included in thefirst electrode 421X.

Note that the layer 423Y containing a light-emitting organic compoundcontains a phosphorescent organic compound, and the light-emittingmodule 450Y emits light Y to the outside. The light Y has a narrowspectral line half-width in which light with a specific wavelength thatis longer than the wavelength of the light mainly included in the lightX is effectively extracted from light emitted from the phosphorescentorganic compound.

The light-emitting module 450Y contains a phosphorescent organiccompound in the layer 423Y containing a light-emitting organic compound.Since excitation lifetime of an excited species of the phosphorescentorganic compound is relatively long, a response time to an input signalis longer than that of a fluorescent organic compound.

Further, with the use of the phosphorescent organic compound, transientcharacteristics of the emission intensity can have a gradual riseportion.

Here, an upper row in FIG. 2D shows transient characteristics of theemission intensity of the light-emitting modules 450X and 450Y when thesignal S0 with a rectangular wave with a rise at the time T0, as shownin the lower row in FIG. 2D, is input to each of the light-emittingmodules 450X and 450Y.

As shown in FIG. 2D, a response time (time until when the emissionintensity reaches 90%) Tx-T0 of the light-emitting module 450X isshorter than a response time Ty-T0 of the light-emitting module 450Y. Inthis manner, response times differ depending on materials oflight-emitting organic compounds. Besides, when thicknesses or materialsof layers included in the layer containing a light-emitting organiccompound differ, depending on differences of electrical characteristicsor electro-optical characteristics, response times may differ.

Light-Emitting Module 450Z

In a pixel 402Z shown in FIG. 2C, a light-emitting module 450Z isprovided. The light-emitting module 450Z includes a light-emittingelement 420Z instead of the light-emitting element 420X in thelight-emitting module 450X. Further, the light-emitting element 420Zincludes a first electrode 421Z and a layer 423Z containing alight-emitting organic compound instead of the first electrode 421X andthe layer 423X containing a light-emitting organic compound in thelight-emitting element 420X. Further, the light-emitting element 420Zincludes a color filter 441Z on a second electrode 422 side of thelight-emitting element 420Z such that the color filter 441Z overlapswith the light-emitting element 420Z.

Note that in the layer 423Z containing a light-emitting organiccompound, a layer 423 a containing a light-emitting organic compound anda layer 423 b containing a light-emitting organic compound are stacked,and the layer 423 a containing a light-emitting organic compound and thelayer 423 b containing a light-emitting organic compound emit light ofcomplementary colors. For example, a layer emitting blue light and alayer emitting yellow light are stacked with an interlayer providedtherebetween. As a result, light emitted from the layer 423Z containinga light-emitting organic compound can have a wide spectrum.

Further, the first electrode 421Z and the second electrode 422 of thelight-emitting module 450Z form a micro resonator, and the color filter441Z is provided to overlap with the micro resonator. With thisstructure, the light-emitting module 450Z emits light Z to the outside.The light Z includes light with a specific wavelength and has a narrowspectral line half-width.

Therefore, in the light-emitting module 450Z, by adjusting the opticalpath length in the micro resonator and the color filter, it is possibleto extract light with a narrow spectral line half-width and with variouscenter wavelengths from the layer 423Z containing a light-emittingorganic compound.

The light-emitting module 450Z may contain a fluorescent organiccompound and a phosphorescent organic compound in the layer 423Zcontaining a light-emitting organic compound. By adjusting the opticalpath length in the micro resonator and the color filter, it is possibleto preferentially extract light emitted from the fluorescent organiccompound having a short response time to an input signal. In a similarmanner, it is also possible to preferentially extract light emitted fromthe phosphorescent organic compound having a long response time to aninput signal.

The above is the description of the light-emitting modules.

Control of Transient Characteristics

When light-emitting modules have different transient characteristics ofthe emission intensity, the transient characteristics of the emissionintensity of the light-emitting modules are preferably controlledindividually. In this case, signals having different voltage waveformsmay be input to the respective light-emitting modules.

FIG. 3A shows changes in transient characteristics of the emissionintensity when a signal S1 x is input to the light-emitting module 450Xhaving a short response time as a light-emitting module. The signal S1 xhas a voltage waveform in which the voltage is a voltage V1 x, which islower than the voltage V0 corresponding to the desired luminance, in aperiod (T0 to T1 x) right after an emission start, as shown in a lowerrow in FIG. 3A.

With the use of the signal S1 x, it is possible to obtain, from thelight-emitting module 450X, light emission having transientcharacteristics in which the rise of the emission intensity in responseto an input signal is gradual.

Further, FIG. 3B shows changes in transient characteristics of theemission intensity when a signal S1 y is input to the light-emittingmodule 450Y having a long response time as a light-emitting module. Thesignal S1 y has a voltage waveform in which the voltage is a voltage V1y, which is lower than the voltage V0 corresponding to the desiredluminance, in a period (T0 to T1 x) right after an emission start, asshown in a lower row in FIG. 3B.

Here, with the use of the voltage V1 y, which is lower than the voltageV1 x, in a period right after the emission start, even from thelight-emitting module 450Y having a long response time, it is possibleto obtain light emission having transient characteristics in which therise of the emission intensity in response to an input signal is moregradual.

In the above manner, by adjusting voltage waveforms of input signals,even when light-emitting modules have different transientcharacteristics of the emission intensity, it is possible to obtainlight emission having substantially the same transient characteristicsof the emission intensity of the light-emitting modules.

Note that as for voltage waveforms of signals input to light-emittingmodules having different response times, not only the value of voltageapplied in the period right after the emission start, but also thelength of time during which a low voltage is applied may differ. Forexample, a time during which a low voltage is applied to alight-emitting module having a short response time is set to a longtime. By thus adjusting the voltage and the length of time individually,it is possible to make transient characteristics from differentlight-emitting modules closer to each other, and to prevent change overtime in the tone of images displayed on the display portion, preventinggeneration of a sense of incompatibility.

The above is the description of the control of the transientcharacteristics.

Structural Example of Display Device

The following shows configuration examples of a display device accordingto one embodiment of the present invention.

FIG. 4 shows a block diagram of a display device 100 shown in thisconfiguration example. The display device 100 includes a display portion101 including a plurality of pixels, a signal line driver circuit 102, ascan line driver circuit 103, a correction control circuit 105, anarithmetic unit 107, and a DA converter 109.

The arithmetic unit 107 is configured to decode a compressed or encodedsignal and output a synchronization signal 201 to the signal line drivercircuit 102 and a video signal which is a digital signal to the DAconverter 109. Similarly, the arithmetic unit 107 is configured tooutput a synchronization signal 202 to the scan line driver circuit 103.

Further, the arithmetic unit 107 may have another function such as apixel interpolation in accordance with upconverting of the resolution,frame interpolation in accordance with upconverting of the framefrequency, or an image processing such as noise removal, grayscaleconversion, or tone correction.

The DA converter 109 converts the digital video signal output from thearithmetic unit 107 into an analog video signal 203 and outputs thevideo signal 203 to the signal line driver circuit 102.

The correction control circuit 105 generates a correction voltage signal204 and a correction synchronization signal 205 from the synchronizationsignal 201 output from the arithmetic unit 107 and the video signal 203output from the DA converter 109, and outputs the correction voltagesignal 204 and the correction synchronization signal 205 to the signalline driver circuit 102.

Note that as a video signal input to the correction control circuit 105,the video signal 203 is used here, which is converted by the DAconverter 109 into an analog signal; however, the digital video signalbefore being input to the DA converter 109 may alternatively be used togenerate the correction voltage signal 204 and the correctionsynchronization signal 205.

The signal line driver circuit 102 and the scan line driver circuit 103drive pixels in the display portion 101 on the basis of thesynchronization signal 201, the synchronization signal 202, the videosignal 203, the correction voltage signal 204, and the correctionsynchronization signal 205, and allows an image to be displayed on thedisplay portion 101.

The display portion 101 can display an image which causes a viewer lessstrain associated with viewing and gives a viewer a great sense ofdepth.

Next, a configuration example of the signal line driver circuit 102 andthe display portion 101 will be described in more detail. FIG. 5 shows aconfiguration example of the signal line driver circuit 102 and thedisplay portion 101.

Here, one pixel including three sub-pixels is shown as the displayportion 101. Each pixel includes a sub-pixel 110R including a redlight-emitting element 111R, a sub-pixel 110G including a greenlight-emitting element 111G, and a sub-pixel 110B including a bluelight-emitting element 111B. Further, transient characteristics of theemission intensity of the light-emitting elements 111R, 111G, and 111Bare different from one another

Note that the description here is made by showing the light-emittingelements included in the sub-pixels emitting any of red light, greenlight, and blue light for simplicity; however, the light-emittingelements can be replaced by the above-described light-emitting modules.

The signal line driver circuit 102 and the display portion 101 areelectrically connected to each other via a signal line 216R, a signalline 216G, and a signal line 216B. Here, the signal line 216R, thesignal line 216G, and the signal line 216B are electrically connected tothe sub-pixel 110R, the sub-pixel 110G, and the sub-pixel 110B,respectively.

In FIG. 5, the sub-pixel 110R including the light-emitting element 111R,the sub-pixel 110E including the light-emitting element 111G, and thesub-pixel 110B including the light-emitting element 111B are providedside by side. Further, each sub-pixel includes a transistor 112, atransistor 113, and a capacitor 114.

In one sub-pixel, the sub-pixel 110R for example (or the sub-pixel 110Gor the sub-pixel 110B), a gate of the transistor 112 is electricallyconnected to a gate line 115, one of a source and a drain of thetransistor 112 is electrically connected to the signal line 216R (or thesignal line 216G or the signal line 216B), and the other of the sourceand the drain of the transistor 112 is electrically connected to oneterminal of the capacitor 114 and a gate of the transistor 113. One of asource and a drain of the transistor 113 is electrically connected to acathode line 116, and the other of the source and the drain of thetransistor 113 is electrically connected to one terminal of thelight-emitting element 111R (or the light-emitting element 111G or thelight-emitting element 111B). The other terminal of the capacitor 114 iselectrically connected to a capacitor line 117. The other terminal ofthe light-emitting element 111R (or the light-emitting element 111G orthe light-emitting element 111B) is electrically connected to an anodeline 118.

Here, to write data to the sub-pixel 110R for example, the transistor112 is turned on by a signal input to the gate line 115, and a potentialof the signal line 216R is given to a node where the gate of thetransistor 113 is connected. In this case, the resistance between thesource and the drain of the transistor 113 is uniquely determined by thepotential given to the gate of the transistor 113. Therefore, voltageapplied to the light-emitting element 111R can be varied by thepotential of the signal line 216R, and the emission intensity of thelight-emitting element 111R can be controlled.

Note that the structure including two transistors and one capacitor isshown here as a structure of the sub-pixel; however, for example, acircuit that corrects variations or changes in characteristics of thetransistor 113 or the light-emitting element can be incorporated.

The signal line driver circuit 102 includes a latch circuit 121 and aplurality of selectors 122R, 122G, and 122B. The selectors 122R, 122G,and 122B are each electrically connected to a selection signal line 211,so that the same selection signal is input to the respective selectorsfrom the latch circuit 121 via the selection signal line 211.

The latch circuit 121 outputs the selection signal to the selectionsignal line 211 on the basis of the synchronization signal 201 inputfrom the arithmetic unit 107. Note that FIG. 5 shows only the selectionsignal line 211 as a selection signal line; actually, the number ofselection signal lines corresponds to the number of pixels provided inthe display portion 101 in the horizontal direction, and the latchcircuit 121 sequentially outputs the selection signal to the selectionsignal lines on the basis of the synchronization signal 201.

The selector 122R is electrically connected to, not only the selectionsignal line 211, but also a wiring 213R to which a red video signal 203Ris input, a wiring 214R to which a correction voltage signal 204R to beinput to the sub-pixel 110R to correct transient characteristics of thered light-emitting element 111R is input, and a wiring 215R to which acorrection synchronization signal 205R for controlling a timing ofoutputting the correction voltage signal 204R to the sub-pixel 110R isinput.

Similarly, the selector 122E is electrically connected to a wiring 213Gto which a green video signal 203G is input, a wiring 214G to which acorrection voltage signal 204G is input, and a wiring 215G to which acorrection synchronization signal 205G is input. Further, the selector122B is electrically connected to a wiring 213E to which a blue videosignal 203B is input, a wiring 214B to which a correction voltage signal204B is input, and a wiring 215B to which a correction synchronizationsignal 205B is input.

In response to the selection signal input from the latch circuit 121 tothe selector 122R, the selector 122R outputs either the video signal203R or the correction voltage signal 204R to the signal line 216R onthe basis of the correction synchronization signal 205R.

Similarly, the selector 122G outputs either the video signal 203G or thecorrection voltage signal 204G to the signal line 216G on the basis ofthe selection signal and the correction synchronization signal 205G.Further, the selector 122B outputs either the video signal 203B or thecorrection voltage signal 204B to the signal line 216B on the basis ofthe selection signal and the correction synchronization signal 205B.

Next, an operation example of the signal line driver circuit 102 will bedescribed with reference to a timing chart shown in FIG. 6.

FIG. 6 shows changes over time in potentials of the selection signalline 211, the video signal 203R, the correction voltage signal 204R, thecorrection synchronization signal 205R, the video signal 203G, thecorrection voltage signal 204G, the correction synchronization signal205G, the video signal 203B, the correction voltage signal 204B, thecorrection synchronization signal 205B, the signal line 216R, the signalline 216G, and the signal line 216B, from the top.

When a high-level potential is input from the latch circuit 121 to theselection signal line 211, the selectors 122R, 122G, and 122B outputsignals to the signal lines 216R, 216G, and 216B, respectively.

Here, the selector 122R outputs a correction voltage signal to thesignal line 216R when the correction synchronization signal 205R is ahigh-level potential, and outputs the video signal 203R to the signalline 216R when the correction synchronization signal 205R is a low-levelpotential.

Therefore, to the signal line 216R, a potential lower than the videosignal 203R is output during a period based on the correctionsynchronization signal 205R right after the selection start, and apotential of the video signal 203R is output after the period. Byinputting such a signal to the sub-pixel 110R, light emission from alight-emitting module including the light-emitting element 111R in thesub-pixel 110R can have transient characteristics in which the rise ofthe emission intensity in response to an input signal is gradual.

Similarly, the selector 122G outputs, to the signal line 216G, a signalhaving a period during which a potential is lower than the video signal203G right after the selection start. Further, the selector 122Boutputs, to the signal line 216B, a signal having a period during whicha potential is lower than the video signal 203B right after theselection start. Therefore, light emission from a light-emitting moduleincluding the light-emitting element 111E in the sub-pixel 110G and alight-emitting module including the light-emitting element 111B in thesub-pixel 110B can also have transient characteristics in which the riseof the emission intensity in response to an input signal is gradual.

When the viewer sees light emission having such transientcharacteristics in which the rise of the emission intensity in responseto an input signal is gradual, stimulation on the brain of the viewer atthe instant of switching of continuous images is relieved. Thus, it ispossible to display an image that is seen as light faithful to thereflected light in nature without strain and that can give a naturalsense of depth.

As described above, by inputting the respective different correctionvoltage signals to the sub-pixels including the light-emitting elementshaving different transient characteristics of the emission intensity,transient characteristics of the emission intensity of thelight-emitting modules in the sub-pixels can be controlled individually.

Further, as shown in FIG. 6, by inputting signals having different pulsewidths as the correction synchronization signals 205R, 205G, and 205B,it is possible to make transient characteristics of the emissionintensity of the sub-pixels closer to each other, and to prevent changein the tone of images displayed on the display portion 101, preventinggeneration of a sense of incompatibility.

When transient characteristics of the emission intensity oflight-emitting elements in two or more sub-pixels are close to eachother, a common correction voltage signal can be used for thesesub-pixels. For example, a common correction voltage signal may be usedfor sub-pixels using light emission from a phosphorescent organiccompound, or sub-pixels using light emission from a fluorescent organiccompound. By using the common correction voltage signal, the number ofwirings can be reduced and a configuration of a correction controlcircuit can be simplified.

When transient characteristics of the emission intensity oflight-emitting elements in two or more sub-pixels are close to eachother, or when two or more sub-pixels can be corrected with only acorrection voltage signal, a common correction synchronization signalcan be used for these sub-pixels. For example, a common correctionsynchronization signal may be used for sub-pixels using light emissionfrom a phosphorescent organic compound, or sub-pixels using lightemission from a fluorescent organic compound. Alternatively, the samecorrection synchronization signal may be used for all the sub-pixelsregardless of structures of light-emitting elements. By using the commoncorrection synchronization signal, the number of wirings can be reducedand a configuration of a correction control circuit can be simplified.

As shown in the above structural example, it is possible to use a signalby which the voltage is raised in a stepwise manner or in a slope as acorrection voltage signal. In this case, the correction control circuitmay output a correction synchronization signal and a correction voltagesignal which are synchronized so that the correction voltage signal canhave a voltage waveform in which the voltage is raised in a stepwisemanner or in a slope to be close to the voltage of a video signal in aperiod during which a high-level potential is given to the correctionsynchronization signal.

Therefore, the display device 100 having the above structure can providean image which gives a viewer a great sense of depth without strain.

The above is the description of the display device 100.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 2

In this embodiment, a structural example of a display device accordingto one embodiment of the present invention and a structural example of adisplay panel which can be used tor the display device according to oneembodiment of the present invention will be described.

Structure of Display Panel

FIGS. 7A to 7C show a structure of a display panel which can be used forthe display device according to one embodiment of the present invention.FIG. 7A is a top view of a structure of a display panel which can beused for the display device according to one embodiment of the presentinvention, FIG. 7B is a side view of a structure including a crosssection taken along line A-B and C-D in FIG. 7A, and FIG. 7C is a sideview of a structure of a pixel including a cross section taken alongline E-F in FIG. 7A.

A display panel 400 shown as an example in this embodiment includes adisplay portion 401 over a first substrate 410. The display portion 401includes a plurality of pixels 402. The pixel 402 includes a pluralityof sub-pixels (e.g., three sub-pixels) (FIG. 7A). Over the firstsubstrate 410, in addition to the display portion 401, a signal linedriver circuit 403 s and a scan line driver circuit 403 g which drivethe display portion 401 are provided. Note that the driver circuits canbe provided not over the first substrate 410 but externally.

The display panel 400 includes an external input terminal and receives avideo signal, a clock signal, a start signal, a reset signal, and thelike from an FPC (flexible printed circuit) 409. Note that although onlyan FPC is illustrated here, a printed wiring board (PWB) may be attachedthereto. The display panel in this specification includes not only amain body of the display panel but one with an FPC or a PWB attachedthereto.

A sealant 405 bonds the first substrate 410 and a second substrate 440.The display portion 401 is sealed in a space 431, formed between thesubstrates (see FIG. 7B).

The structure including the cross section of the display panel 400 isdescribed with reference to FIG. 7B. The display panel 400 includes thesignal line driver circuit 403 s, a sub-pixel 402G included in the pixel402, and a lead wiring 408. Note that the display portion 401 of thedisplay panel 400 shown as an example in this embodiment emits light inthe direction denoted by the arrow in the drawing, thereby displayingimages.

A CMOS circuit, which is a combination of an n-channel transistor 413and a p-channel transistor 414, is formed for the signal line drivercircuit 403 s. Note that the driver circuit is not limited to thisstructure and may be various circuits, such as a CMOS circuit, a PMOScircuit, or an NMOS circuit.

The lead wiring 408 transmits a signal inputted from an external inputterminal to the signal line driver circuit 403 s and the scan linedriver circuit 403 g.

The sub-pixel 402G includes a switching transistor 411, a currentcontrol transistor 412, and a light-emitting module 450G. Note that aninsulating layer 416 and a partition 418 are formed over the transistor411 and the like. The light-emitting module 450G includes a reflectivefilm, a semi-transmissive and semi-reflective film, a light-emittingelement 420G between the reflective film and the semi-transmissive andsemi-reflective film, and a color filter 441G provided on thesemi-transmissive and semi-reflective film side through which lightemitted from the light-emitting element 420G is extracted. In thelight-emitting module 450G shown as an example in this embodiment, afirst electrode 421G and a second electrode 422 of the light-emittingelement 420G also serve as the reflective film and the semi-transmissiveand semi-reflective film, respectively. Note that a direction of animage displayed in the display portion 401 is determined in accordancewith a direction in which light emitted from the light-emitting element420G is extracted.

In addition, a light-blocking film 442 is formed so as to surround thecolor filter 441G. The light-blocking film 442 prevents a phenomenon inwhich the display panel 400 reflects outside light and has an effect ofincreasing the contrast of images displayed in the display portion 401.Note that the color filter 441G and the light-blocking film 442 areformed on the second substrate 440.

The insulating layer 416 is a layer having insulating properties forplanarizing a step due to the structure of the transistor 411 and thelike or for suppressing impurity dispersion to the transistor 411 andthe like. The insulating layer 416 can be a single layer or a stackedlayer. The partition 418 is an insulating layer having an opening; thelight-emitting element 420G is formed in the opening of the partition418.

The light-emitting element 420G includes the first electrode 421G, thesecond electrode 422, and a layer 423 containing a light-emittingorganic compound.

Structural Example of Transistor

Top-gate transistors are used in the display panel 400 shown as anexample in FIG. 7A. Various types of transistors can be used for thesignal line driver circuit 403 s, the scan line driver circuit 403 g,and the sup-pixels. Note that various semiconductors can be used for aregion where channels of these transistors are formed. Specifically, aswell as amorphous silicon, polysilicon, or single crystal silicon, anoxide semiconductor or the like can be used.

When an oxide semiconductor is used for a region where a channel of atransistor is formed, the transistor can be smaller than a transistor inwhich an amorphous silicon is used, which results in higher resolutionpixels in a display portion.

When a single crystal semiconductor is used for a region where a channelof a transistor is formed, the size of the transistor can be reduced,which results in even higher resolution pixels in a display portion.

As a single crystal semiconductor used for forming a semiconductorlayer, a semiconductor substrate, typical examples of which include asingle crystal semiconductor substrate formed using elements belongingto Group 14, such as a single crystal silicon substrate, a singlecrystal germanium substrate, or a single crystal silicon germaniumsubstrate, and a compound semiconductor substrate (e.g., a SiCsubstrate, a sapphire substrate, and a GaN substrate), can be used.Preferred one is a silicon on insulator (SOI) substrate in which asingle crystal semiconductor layer is provided on an insulating surface.

An SOI substrate can be fabricated by the following method: after oxygenions are implanted in a mirror-polished wafer, the wafer is heated athigh temperatures to form an oxidized layer at a predetermined depthfrom a surface of the wafer and eliminate defects generated in a surfacelayer. Alternatively, an SOI substrate can be fabricated by the methodin which the semiconductor substrate is separated by utilizing thegrowth of microvoids formed by hydrogen ion irradiation (this growth iscaused by heat treatment). Alternatively, an SOI substrate can befabricated by the method in which a single crystal semiconductor layeris formed on an insulating surface by crystal growth.

In this embodiment, ions are added through one surface of a singlecrystal semiconductor substrate, an embrittlement layer is formed at apredetermined depth from the one surface of the single crystalsemiconductor substrate, and an insulating layer is formed over the onesurface of the single crystal semiconductor substrate or over the firstsubstrate 410. Next, heat treatment is performed in the state in whichthe single crystal semiconductor substrate provided with theembrittlement layer and the first substrate 410 are bonded to each otherwith the insulating layer interposed therebetween, so that a crack isgenerated in the embrittlement layer to separate the single crystalsemiconductor substrate along the embrittlement layer. Thus, a singlecrystal semiconductor layer, which is separated from the single crystalsemiconductor substrate, is formed as a semiconductor layer over thefirst substrate 410. Note that a glass substrate can be used as thefirst substrate 410.

Further, regions electrically insulated from each other may be formed inthe semiconductor substrate so that transistors 411 and 412 may beformed using the regions electrically insulated from each other.

By forming a channel formation region using a single crystalsemiconductor, variations in electrical characteristics, such asthreshold voltage, between transistors due to bonding defects at grainboundaries can be reduced. Thus, in the panel according to oneembodiment of the present invention, the light-emitting elements can beoperated normally without placing a circuit for compensating thresholdvoltage in each pixel. The number of circuit elements per pixel cantherefore be reduced, increasing the flexibility in layout. Thus, ahigh-definition display panel can be achieved. For example, a structurein which a matrix of a plurality of pixels is included, specifically 300or more pixels per inch (i.e., the horizontal resolution is 300 or morepixels per inch (ppi)), preferably 400 or more pixels per inch (i.e.,the horizontal resolution is 400 or more ppi), can be achieved.

Moreover, a transistor whose channel formation region is composed of asingle crystal semiconductor can be downsized while keeping high currentdrive capability. The use of the downsized transistor leads to areduction in the area of a circuit portion that does not contribute todisplay, which results in an increase in the display area in the displayportion and a reduction in the frame size of the display panel.

Configuration of Pixel

A structure of the pixel 402 included in the display portion 401 isdescribed with reference to FIG. 7C.

The pixel 402 shown as an example in this embodiment includes thesub-pixel 402G. The sub-pixel 402G includes the light-emitting element420G; the light-emitting element 420G includes the first electrode 421Galso serving as a reflective film, the second electrode 422 also servingas a semi-transmissive and semi-reflective film, a layer 423 acontaining a light-emitting organic compound, a layer 423 b containing alight-emitting organic compound, and an interlayer 424. Further, thepixel 402 includes the color filter 441G on the second electrode 422side so that the color filter 441G may overlap with the light-emittingelement 420G, and light with a spectral line half-width of 60 nm or lessand wavelengths of greater than or equal to 400 nm and less than 800 nmis emitted. Furthermore, it is possible to obtain, as light emission ofthe light-emitting module 450G, light emission having transientcharacteristics in which the rise of the emission intensity in responseto an input signal is gradual.

When the viewer sees light emission having such transientcharacteristics in which the rise of the emission intensity in responseto an input signal is gradual, stimulation on the brain of the viewer atthe instant of switching of continuous images is relieved. Thus, it ispossible to display an image that is seen as light faithful to thereflected light in nature without strain and that can give a naturalsense of depth.

Such a pixel is provided in the display portion 401 at a resolution of80 ppi or higher, preferably 300 ppi or higher, and a display devicewith an NTSC ratio of 80% or higher, preferably 95% or higher, and acontrast ratio of 500 or higher, preferably 2000 or higher is provided.Consequently, it is possible to provide a display device that displaysan image which causes a viewer less strain associated with viewing andgive a viewer a great sense of depth. In addition, interference of lightemitted from the light-emitting element occurs between the reflectivefilm and the semi-transmissive and semi-reflective film, specific lightamong light with a wavelength of greater than or equal to 400 am andless than 800 nm is strengthened, and unnecessary light is absorbed bythe color filter. Accordingly, high color saturation images can bedisplayed by light with a narrow spectral line width (specifically, aspectral line half-width of 60 nm or less), thereby giving a viewer agreater sense of depth.

Therefore, a display device having the above structure can provide animage which gives a viewer a great sense of depth without strain.

In addition, the pixel 402 includes a sub-pixel 402B emitting blue lightB, the sub-pixel 402G emitting green light G, and a sub-pixel 402Remitting red light R. Each sub-pixel includes a driver transistor and alight-emitting module. Each light-emitting module includes a reflectivefilm, a semi-transmissive and semi-reflective film, and a light-emittingelement between the reflective film and the semi-transmissive andsemi-reflective film.

When a microresonator is formed by making an overlap between thereflective film and the semi-transmissive and semi-reflective film and alight-emitting element is formed therebetween, light with a specificwavelength can be efficiently extracted through the semi-transmissiveand semi-reflective film. Specifically, the optical path length of themicroresonator is n/2 times (n is a natural number) the wavelength ofextracted light; thus, light extraction efficiency can be enhanced. Thewavelength of extracted light depends on the distance between thereflective film and the semi-transmissive and semi-reflective film, andthe distance can be adjusted by forming an optical adjustment layerbetween the films.

A conductive film having light-transmitting properties to visible lightor a layer containing a light-emitting organic compound can be employedfor a material that can be used for the optical adjustment layer. Forexample, the thickness of the optical adjustment layer may be adjustedusing a charge generation region. Alternatively, a region containing asubstance having a high hole-transport property and an acceptorsubstance is preferably used for the optical adjustment layer because anincrease in driving voltage can be suppressed even when the opticaladjustment layer is thick.

As the structure of the light-emitting element, the light-emittingelement 420G is provided between the first electrode 421G also servingas a reflective film and the second electrode 422 also serving as asemi-transmissive and semi-reflective film. The light-emitting element420G includes the layer 423 a containing a light-emitting organiccompound, the layer 423 h containing a light-emitting organic compound,and the interlayer 424.

Note that the structural example of the light-emitting element will bedescribed in detail in Embodiment 3.

Here, in the case of a display device using a liquid-crystal element ina pixel, the response time cannot be shortened enough because an imageis displayed by physically changing the orientation of liquid crystals.In contrast, the response time of the above-described light-emittingelement is faster than that of a liquid-crystal element. Thus, a displaydevice using such a light-emitting element can display smooth movingimages, in which afterimages do not likely appear when displaying movingimages. As a result, a display device capable of displaying more vividand stereoscopic images and giving viewers a great sense of depth can beobtained.

The light-emitting modules shown as an example in this embodiment eachhave a structure in which the second electrode 422 provided in thelight-emitting module also serves as a semi-transmissive andsemi-reflective film. Specifically, the second electrode 422 shared bythe light-emitting elements 420B, 420G, and 420R also serves as asemi-transmissive and semi-reflective film of the light-emitting modules450B, 450G, and 450R.

In addition, the light-emitting element is provided in an electricallyseparate manner in each light-emitting module, and the first electrodeof the light-emitting element also serves as a reflective film.Specifically, a first electrode 421B provided in the light-emittingelement 420B also serves as a reflective film of the light-emittingmodule 450B, the first electrode 421G provided in the light-emittingelement 420G also serves as a reflective film of the light-emittingmodule 450G, and a first electrode 421R provided in the light-emittingelement 420R also serves as a reflective film of the light-emittingmodule 450R.

The first electrode also serving as a reflective film of alight-emitting module has a stacked-layer structure in which an opticaladjustment layer is stacked over the reflective film. The opticaladjustment layer is preferably formed of a conductive film havinglight-transmitting properties with respect to visible light, and thereflective film is preferably formed of a conductive metal film havinghigh reflectivity with respect to visible light.

The thickness of the optical adjustment layer is adjusted in accordancewith a wavelength of light extracted from a light-emitting module.

For example, the first light-emitting module 450B includes a colorfilter 441B which transmits blue light, the first electrode 421B alsoserving as a reflective film, and the second electrode 422 also servingas a semi-transmissive and semi-reflective film; the optical path lengthbetween the first electrode 421B and the second electrode 422 isadjusted to k/2 times (k is a natural number) a length greater than orequal to 400 nm and less than 500 nm.

Further, the second light-emitting module 450G includes the color filter441G which transmits green light, a reflective film, and asemi-transmissive and semi-reflective film; the optical path lengthbetween the reflective film and the semi-transmissive andsemi-reflective film is adjusted to j/2 times (j is a natural number) alength greater than or equal to 500 nm and less than 600 nm.

Further, the third light-emitting module 450R includes a color filter441R which transmits red light, a reflective film, and asemi-transmissive and semi-reflective film; the optical path lengthbetween the reflective film and the semi-transmissive andsemi-reflective film is adjusted to i/2 times (i is a natural number) alength greater than or equal to 600 nm and less than 800 nm.

In such a light-emitting module, interference of light emitted from thelight-emitting elements occurs between the reflective film and thesemi-transmissive and semi-reflective film, light having a specificwavelength among light having a wavelength of greater than or equal to400 nm and less than 800 nm is strengthened, and unnecessary light isabsorbed by the color filter. Accordingly, high color saturation imagescan be displayed by light with a narrow spectral line width(specifically, a spectral line half-width of 60 nm or less), therebygiving a viewer a greater sense of depth. Consequently, it is possibleto provide a display device that can display images which cause a viewerless strain associated with viewing and give a viewer a great sense ofdepth.

In particular, the third light-emitting module 450R emits red light witha spectral line half-width of less than 50 nm, the second light-emittingmodule 450G emits green light with a spectral line half-width which isnarrower than that of the light emitted from the third light-emittingmodule 450R, and the first light-emitting module 450B emits blue lightwith a spectral line half-width which is narrower than that of the lightemitted from the second light-emitting module 450G.

In the light-emitting module with such a structure, the half-width ofthe green light whose luminosity factor is higher than that of the redlight is narrower than the half-width of the red light, and thehalf-width of the blue light is narrower than the half-width of thegreen light. Thus, an image with high saturation can be displayed withthe use of light with a narrow spectral line width (specifically; aspectral line half-width of 50 nm or less), and a depth effect isenhanced.

Note that the first light-emitting module 450B, the secondlight-emitting module 450G, and the third light-emitting module 450Reach include the layer 423 a containing a light-emitting organiccompound, the layer 423 b containing a light-emitting organic compound,and the interlayer 424. In addition, one of the pair of electrodes ofthe light-emitting element also serves as a reflective film and theother thereof also serves as a semi-transmissive and semi-reflectivefilm.

In the light-emitting modules with such a structure, each layercontaining a light-emitting organic compound in the plurality oflight-emitting modules can be formed in one process. Further, the pairof electrodes also serves as the reflective film and thesemi-transmissive and semi-reflective film. Therefore, a manufacturingprocess can be simplified. Thus, it is possible to provide a displaydevice that can display an image which causes a viewer less strainassociated with viewing and gives a viewer a great sense of depth.

Structure of Partition

The partition 418 is formed to cover end portions of the firstelectrodes 421B, 421G, and 421R.

The partition 418 has a curved surface with curvature at a lower endportion thereof. As a material of the partition 418, negative orpositive photosensitive resin can be used.

Note that using a material absorbing visible light for the partitionproduces an effect of suppressing light leakage into adjacentlight-emitting elements (also called cross talk).

In addition, in a structure that displays images by extracting lightemitted from the light-emitting module from the first substrate 410 sidewhich is provided with a semi-transmissive and semi-reflective film, thepartition formed using a material absorbing visible light absorbsoutside light which is reflected by the reflective film on the firstsubstrate 410, thereby suppressing the reflection.

Sealing Structure

The display panel 400 shown as an example in this embodiment has astructure in which the light-emitting element is sealed in a spaceenclosed by the first substrate 410, the second substrate 440, and thesealant 405.

The space can be filled with an inert gas (e.g., nitrogen or argon) orresin. An absorbent of impurity (typically, water and/or oxygen) such asa thy agent may be provided.

The sealant 405 and the second substrate 440 are desirably formed usinga material which does not transmit impurities in the air (such as waterand/or oxygen) as much as possible. An epoxy-based resin, glass frit, orthe like can be used for the sealant 405.

Examples of the second substrate 440 include a glass substrate; a quartzsubstrate; a plastic substrate formed of polyvinyl fluoride (PVF),polyester, an acrylic resin, or the like; a substrate offiberglass-reinforced plastics (FRP); and the like.

Modification Example

FIGS. 8A and 8B show a modification example of this embodiment. FIG. 8Ais a side view of a structure including cross sections taken along lineA-B and C-D in FIG. 7A, and FIG. 8B is a side view of a structure of apixel including a cross section taken along line E-F in FIG. 7A.

A display panel shown in FIGS. 8A and 8B is a modification example ofthe display panel shown in FIGS. 7A to 7C and has a pixel structuredifferent from that in the display panel shown in FIGS. 7A to 7C.Specifically, the display panel shown in FIGS. 8A and 8B is differentfrom the display panel shown in FIGS. 7A to 7C in that the color filteris not provided and in that sub-pixels of different emission colorsinclude layers containing different light-emitting organic compounds. Amodification example of a structure of the pixel 402 included in thedisplay portion 401 is described with reference to FIG. 8B.

The pixel 402 shown as the modification example in this embodimentincludes the sub-pixel 402B emitting blue light B, the sub-pixel 402Gemitting green light G, and the sub-pixel 402R emitting red light R.Each sub-pixel includes a driver transistor and a light-emitting module.Each light-emitting module includes a reflective film, asemi-transmissive and semi-reflective film, and a light-emitting elementbetween the reflective film and the semi-transmissive andsemi-reflective film.

The sub-pixel 402B includes the first electrode 421B also serving as thereflective film, the second electrode 422 also serving as thesemi-transmissive and semi-reflective film, and a layer 423B containinga light-emitting organic compound that emits light including blue light.Further, the optical path length in the micro resonator is adjusted suchthat blue light with a spectral line half-width of less than or equal to60 nm is emitted.

The sub-pixel 402G includes the first electrode 421G also serving as thereflective film, the second electrode 422 also serving as thesemi-transmissive and semi-reflective film, and a layer 423G containinga light-emitting organic compound that emits light including greenlight. Further, the optical path length in the micro resonator isadjusted such that green light with a spectral line half-width of lessthan or equal to 60 nm is emitted.

The sub-pixel 402R includes the first electrode 421R also serving as thereflective film, the second electrode 422 also serving as thesemi-transmissive and semi-reflective film, and a layer 423R containinga light-emitting organic compound that emits light including red light.Further, the optical path length in the micro resonator is adjusted suchthat red light with a spectral line half-width of less than or equal to60 nm is emitted.

Further, as light emission from any one of the light-emitting module450B, the light-emitting module 450G, and the light-emitting module450R, it is possible to obtain light emission having transientcharacteristics in which the rise of the emission intensity in responseto an input signal is gradual.

Note that materials that can be used for the layers containing therespective light-emitting organic compounds will be described in detailin Embodiment 3.

Such a pixel is provided in the display portion 401 at a resolution of80 ppi or higher, preferably 300 ppi or higher, and a display devicewith an NTSC ratio of 80% or higher, preferably 95% or higher, and acontrast ratio of 500 or higher, preferably 2000 or higher is provided.Consequently, it is possible to provide a display device that displaysan image which causes a viewer less strain associated with viewing andgive a viewer a great sense of depth. In addition, interference of lightemitted from the light-emitting element occurs between the reflectivefilm and the semi-transmissive and semi-reflective film, and specificlight among light with a wavelength of greater than or equal to 400 nmand less than 800 nm is strengthened. Accordingly, high color saturationimages can be displayed by light with a narrow spectral line width(specifically, a spectral line half-width of 60 nm or less), therebygiving a viewer a greater sense of depth.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 3

In this embodiment, a structure of a display panel which can be used fora display device according to one embodiment of the present inventionwill be described.

FIG. 9A is a side view of a structure including a cross section takenalong line A-B and C-D in FIG. 7A, and FIG. 9B is a side view of astructure including a cross section taken along line A-B and C-D in FIG.7A.

A display panel shown as an example in FIG. 9A or FIG. 9B has the sametop surface structure as the display panel shown as an example inEmbodiment 2, but has a different side surface structure from that ofthe display panel shown as an example in Embodiment 2. Note thatportions having the same structure as those described in Embodiment 2are denoted by the same reference numerals, and the description ofEmbodiment 2 is applied thereto.

Structural Example 1 of Display Panel

In the display panel shown as an example in FIG. 9A, a display portionincluding the sub-pixel 402G and the signal line driver circuit 403 sare provided over the first substrate 410. A transistor 471 is providedin the sub-pixel 402G, and a transistor 472 is provided in the signalline driver circuit 403 s. Both of the transistors 471 and 472 arebottom-gate transistors.

A second gate electrode (also referred to as a back gate) may beprovided to overlap with a semiconductor of a region in the transistorwhere a channel is formed. The characteristics (e.g., threshold voltage)of the transistor provided with the second gate electrode can becontrolled by a potential to be given to the second gate electrode.

A pair of spacers 445 is provided over a partition 418, therebycontrolling a gap between the first substrate 410 and the secondsubstrate 440. Thus, it is possible to prevent a problem ofdisfigurement in which patterns (also called Newton's rings) derivedfrom optical interference between the first substrate 410 and the secondsubstrate 440 are observed. Further, it is possible to prevent opticalcrosstalk by providing the pair of spacers 445 such that light leakagefrom the adjacent sub-pixel is prevented.

The following will show an example of a semiconductor which ispreferably used for the region in the transistor where a channel isformed, as an example in this embodiment.

An oxide semiconductor has a wide energy gap of 3.0 eV or more. Atransistor including an oxide semiconductor film obtained by processingof the oxide semiconductor in an appropriate condition and a sufficientreduction in carrier density of the oxide semiconductor can have muchlower leakage current between a source and a drain in an off state(off-state current) than a conventional transistor including silicon.

An oxide semiconductor containing at least indium (In) or zinc (Zn) ispreferably used. In particular, In and Zn are preferably contained. As astabilizer for reducing variation in electric characteristics of atransistor using the oxide semiconductor, gallium (Ga) is preferablyadditionally contained. Tin (Sn) is preferably contained as astabilizer. In addition, as a stabilizer one or more selected fromhafnium (Hf), zirconium (Zr), titanium (Ti), scandium (Sc), yttrium (Y),and a lanthanoid element (such as cerium (Ce), neodymium (Nd), orgadolinium (Gd), for example) is preferably contained.

As the oxide semiconductor, for example, any of the following can beused: indium oxide, tin oxide, zinc oxide, an In—Zn-based oxide, aSn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, an In—Ga-based oxide, anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide,an In—Zr—Zn-based oxide, an In—Ti—Zn-based oxide, an In—Sc—Zn-basedoxide, an In—Y—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-basedoxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, and anIn—Hf—Al—Zn-based oxide.

Here, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Znas its main components and there is no particular limitation on theratio of In to Ga and Zn. The In—Ga—Zn-based oxide may contain a metalelement other than the In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 issatisfied, and m is not an integer) may be used as an oxidesemiconductor. Note that M represents one or more metal elementsselected from Ga, Fe, Mn, and Co, or the above-described element as astabilizer. Further alternatively, as the oxide semiconductor, amaterial expressed by a chemical formula, In₂SnO₅(ZnO)_(n) (n>0, n is aninteger) may be used.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1, In:Ga:Zn=3:1:2, or In:Ga:Zn=2:1:3, or any of oxideswhose composition is in the neighborhood of the above compositions ispreferably used.

An oxide semiconductor film which can be used for a semiconductor layerof a transistor may be in a single crystal state or a non-single-crystalstate. The non-single-crystal state is, for example, structured by atleast one of c-axis aligned crystal (CAAC), polycrystal, microcrystal,and an amorphous part. The density of defect states of an amorphous partis higher than those of microcrystal and CAAC. The density of defectstates of microcrystal is higher than that of CAAC. Note that an oxidesemiconductor including CAAC is referred to as a CAAC-OS (c-axis alignedcrystalline oxide semiconductor).

The oxide semiconductor film is preferably a CAAC-OS film.

For example, the oxide semiconductor film may include microcrystal. Notethat an oxide semiconductor including microcrystal is referred to as amicrocrystalline oxide semiconductor. A microcrystalline oxidesemiconductor film includes microcrystal (also referred to asnanocrystal) with a size of greater than or equal to 1 nm and less than10 nm, for example.

For example, the oxide semiconductor film may include an amorphous part.Note that an oxide semiconductor including an amorphous part is referredto as an amorphous oxide semiconductor. An amorphous oxide semiconductorfilm, for example, has disordered atomic arrangement and no crystallinecomponent. Alternatively, an amorphous oxide semiconductor film is, forexample, absolutely amorphous and has no crystal part.

Note that the oxide semiconductor film may be a mixed film including anyof a CAAC-OS, a microcrystalline oxide semiconductor, and an amorphousoxide semiconductor. The mixed film, for example, includes a region ofan amorphous oxide semiconductor, a region of a microcrystalline oxidesemiconductor, and a region of a CAAC-OS. Further, the mixed film mayhave a stacked structure including a region of an amorphous oxidesemiconductor, a region of a microcrystalline oxide semiconductor, and aregion of a CAAC-OS, for example.

The oxide semiconductor film may be in a single-crystal state, forexample.

An oxide semiconductor film preferably includes a plurality of crystalparts. In each of the crystal parts, a c-axis is preferably aligned in adirection parallel to a normal vector of a surface where the oxidesemiconductor film is formed or a normal vector of a surface of theoxide semiconductor film. Note that, among crystal parts, the directionsof the a-axis and the b-axis of one crystal part may be different fromthose of another crystal part. An example of such an oxide semiconductorfilm is a CAAC-OS film.

A CAAC-OS film is described below.

In most cases, a crystal part of the CAAC-OS film fits inside a cubewhose one side is less than 100 nm. In an image obtained with atransmission electron microscope (TEM), a boundary between crystal partsin the CAAC-OS film is not clearly detected. Further, with the TEM, agrain boundary in the CAAC-OS film is not clearly detected. Thus, in theCAAC-OS film, a reduction in electron mobility, due to the grainboundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, for example,a c-axis is aligned in a direction parallel to a normal vector of asurface where the CAAC-OS film is formed or a normal vector of a surfaceof the CAAC-OS film. Further, in each of the crystal parts, metal atomsare arranged in a triangular or hexagonal configuration when seen fromthe direction perpendicular to the a-b plane, and metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis. Note that, among crystal parts, the directions of thea-axis and the b-axis of one crystal part may be different from those ofanother crystal part. In this specification, a term “perpendicular”includes a range from 80° to 100°, preferably from 85° to 95°. Inaddition, a term “parallel” includes a range from −10° to 10°,preferably from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystal part in a region to which the impurity is added may have lowcrystallinity.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note that thefilm deposition is accompanied with the formation of the crystal partsor followed by the formation of the crystal parts throughcrystallization treatment such as heat treatment. Hence, the c-axes ofthe crystal parts are aligned in the direction parallel to a normalvector of the surface where the CAAC-OS film is formed or a normalvector of the surface of the CAAC-OS film.

There are three methods for forming a CAAC-OS film when the CAAC-OS filmis used as the oxide semiconductor film.

The first method is to form an oxide semiconductor film at a temperaturehigher than or equal to 100° C. and lower than or equal to 600° C. toform, in the oxide semiconductor film, crystal parts in which the c-axesare aligned in the direction parallel to a normal vector of a surfacewhere the oxide semiconductor film is formed or a normal vector of asurface of the oxide semiconductor film.

The second method is to form an oxide semiconductor film with a smallthickness and then heat it at a temperature higher than or equal to 200°C. and lower than or equal to 700° C., to form, in the oxidesemiconductor film, crystal parts in which the c-axes are aligned in thedirection parallel to a normal vector of a surface where the oxidesemiconductor film is formed or a normal vector of a surface of theoxide semiconductor film.

The third method is to form a first oxide semiconductor film with asmall thickness, then heat it at a temperature higher than or equal to200° C. and lower than or equal to 700° C., and form a second oxidesemiconductor film, to form, in the oxide semiconductor film, crystalparts in which the c-axes are aligned in the direction parallel to anormal vector of a surface where the oxide semiconductor film is formedor a normal vector of a surface of the oxide semiconductor film.

In the case where a CAAC-OS film is deposited by a sputtering method forexample, a substrate temperature in the deposition is preferably high.For example, an oxide film is deposited at a substrate heatingtemperature from 100° C. to 600° C., preferably from 200° C. to 500° C.,further preferably from 150° C. to 450° C., whereby a CAAC-OS film canbe deposited.

Electric power used in a sputtering method is preferably supplied from adirect-current (DC) source. Note that a radio frequency (RF) powersource or an alternating-current (AC) power source can be used. Notethat it is difficult to use an RF power source for a sputteringapparatus which is capable of deposition to a large-area substrate. Inaddition, a DC power source is preferred to an AC power source in thefollowing respect.

In the case where an In—Ga—Zn—O compound target is used as a sputteringtarget, an In—Ga—Zn—O compound target in which InOx powder, GaOy powder,and ZnOz powder are mixed in the molar ratio of 2:2:1, 8:4:3, 3:1:1,1:1:1, 4:2:3, 3:1:2, 3:1:4, 1:6:4, 1:6:9, or the like is preferablyused, for example. Note that x, y, and z are any positive numbers.Further, a sputtering target may be polycrystalline.

Alternatively, with use of magnetron, the density of a plasma area neara sputtering target can be increased by a magnetic field. For example,in a magnetron sputtering apparatus, a magnetic assembly is located inthe back of the sputtering target and a magnetic field is generated inthe front of the sputtering target. When sputtering to the sputteringtarget, the magnetic field traps ionized electrons and secondaryelectrons generated by the sputtering. The electrons trapped in this wayenhance the odds of collision with an inert gas, such as a rare gas, inthe deposition chamber, thereby increasing the plasma density. Thus, thedeposition rate can be increased without significantly increasing thetemperature of an element formation layer.

In the case where a CAAC-OS film is deposited by a sputtering method,for example, impurities (e.g., hydrogen, water, carbon dioxide, andnitrogen) existing in a deposition chamber of a sputtering apparatus ispreferably reduced. Further, the concentration of impurities in adeposition gas is preferably reduced. For example, as a deposition gassuch as an oxygen gas or an argon gas, a highly purified gas having adew point of −40° C. or lower, preferably −80° C. or lower, stillpreferably −100° C. or lower is used, whereby suppressing entry ofimpurities into a CAAC-OS film.

In the case where a CAAC-OS film is deposited by a sputtering method, itis preferable to suppress plasma damage when the deposition is performedby increasing the oxygen percentage in the deposition gas and optimizingelectric power. For example, the oxygen percentage in the deposition gasis preferably 30 vol % or higher, still preferably 100 vol %.

In the case where a CAAC-OS film is deposited by a sputtering method,heat treatment may be performed in addition to the substrate heatingwhen the deposition is performed. By the heat treatment, the impurityconcentration in the oxide semiconductor film can be reduced, forexample.

The heat treatment is performed at higher than or equal to 350° C. andlower than a strain point of the substrate, or may be performed athigher than or equal to 350° C. and lower than or equal to 450° C. Notethat the heat treatment may be performed more than once.

There is no particular limitation on a heat treatment apparatus to beused for the heat treatment, and a rapid thermal annealing (RTA)apparatus such as a gas rapid thermal annealing (GRTA) apparatus or alamp rapid thermal annealing (LRTA) apparatus may be used.Alternatively, another heat treatment apparatus such as an electricfurnace may be used.

As described in the above process, an impurity concentration in theoxide semiconductor film is reduced by preventing entry of hydrogen,water, or the like into the film during the deposition. The impurityconcentration can be reduced by removing hydrogen, water, or the likecontained in the oxide semiconductor film through the heat treatmentafter the deposition of the oxide semiconductor film. After that, oxygenis supplied to the oxide semiconductor film to repair oxygen defects,thereby highly purifying the oxide semiconductor film. Moreover, oxygenmay be added to the oxide semiconductor film.

With the use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small. Thus, the transistor has highreliability.

The above is the description of the CAAC-OS film.

After formation of the oxide semiconductor film, it is preferable thatdehydration treatment (dehydrogenation treatment) be performed to removehydrogen or moisture from the oxide semiconductor film so that the oxidesemiconductor film is highly purified to contain impurities as little aspossible, and that oxygen be added to the oxide semiconductor film tofill oxygen vacancies increased by the dehydration treatment(dehydrogenation treatment). In this specification and the like,supplying oxygen to an oxide semiconductor film may be expressed asoxygen adding treatment or treatment for making the oxygen content of anoxide semiconductor film be in excess of that in the stoichiometriccomposition may be expressed as treatment for making an oxygen-excessstate.

In this manner, hydrogen or moisture is removed from the oxidesemiconductor film by dehydration treatment (dehydrogenation treatment)and oxygen vacancies therein are filled by oxygen adding treatment,whereby the oxide semiconductor film can be turned into an i-type(intrinsic) or substantially i-type oxide semiconductor film. The oxidesemiconductor film which is highly purified in such a manner includesextremely few (close to zero) carriers derived from a donor, and thecarrier concentration thereof is lower than 1×10¹⁷/cm³, lower than1×10¹⁵/cm³, lower than 1×10¹⁴/cm³, lower than 1×10¹³/cm³, lower than1×10¹²/cm³, lower than 1×10¹¹/cm³, or lower than 1.45×10¹⁰/cm³.

The transistor including the oxide semiconductor film which is highlypurified by sufficiently reducing the hydrogen concentration, and inwhich defect levels in the energy gap due to oxygen vacancies arereduced by sufficiently supplying oxygen can achieve extremely excellentoff-state current characteristics. For example, the off-state current(per unit channel width (1 μm) here) at room temperature (25° C.) isless than or equal to 100 yA (1 yA (yoctoampere) is 1×10⁻²⁴ A),desirably less than or equal to 10 yA. In addition, the off-statecurrent (per unit channel width (1 mm), here) at 85° C. is less than orequal to 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A), desirably less thanor equal to 10 zA. In this manner, the transistor which has extremelyexcellent off-state current characteristics can be obtained with the useof an i-type (intrinsic) or substantially i-type oxide semiconductorfilm.

Structural Example 2 of Display Panel

In the display panel shown as an example in FIG. 9B, a bottom-gatetransistor is used. A light-emitting module provided for a pixel in thedisplay portion has such a structure as to emit light to the firstsubstrate 410 side.

Specifically, the first electrode 421G of the light-emitting element420G in the light-emitting module 450G also serves as asemi-transmissive and semi-reflective film, and the second electrode 422also serves as a reflective film. Thus, light emitted from thelight-emitting element 420G is extracted from the first substrate 410through a color filter 428G provided between the first electrode 421Gand the first substrate 410. In other words, the light-emitting element420G in the light-emitting module 450G can be referred to as abottom-emission light-emitting element.

The color filter 428G is formed over the first substrate 410 over whicha transistor 481 is provided. A light-blocking film 429 is formed tosurround the color filter 428G.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 4

In this embodiment, a structure of a light-emitting element which can beused for the light-emitting module according to one embodiment of thepresent invention will be described with reference to FIGS. 10A to 10C.

The light-emitting element shown as an example in this embodimentincludes a first electrode, a second electrode, and a layer containing alight-emitting organic compound (hereinafter referred to as an EL layer)provided between the first electrode and the second electrode. Note thatone of the first electrode and the second electrode functions as ananode, and the other functions as a cathode. The EL layer is providedbetween the first electrode and the second electrode, and a structure ofthe EL layer may be appropriately selected in accordance with materialsof the first electrode and second electrode.

Structural Example of Light-Emitting Element

An example of a structure of the light-emitting element is illustratedin FIG. 10A. The light-emitting element shown as an example in FIG. 10Aincludes an EL layer formed of a first light-emitting unit 1103 a and asecond light-emitting unit 1103 b between an anode 1101 and a cathode1102. Furthermore, an interlayer 1104 is provided between the firstlight-emitting unit 1103 a and the second light-emitting unit 1103 b.

When voltage higher than the threshold voltage of the light-emittingelement is applied between the anode 1101 and the cathode 1102, holesare injected to the EL layer from the anode 1101 side and electrons areinjected to the EL layer from the cathode 1102 side. The injectedelectrons and holes are recombined in the EL layer, so that alight-emitting substance contained in the EL layer emits light.

Note that in this specification, a layer or a stacked body whichincludes one region where electrons and holes injected from both endsare recombined is referred to as a light-emitting unit.

Note that the number of the light-emitting units provided between theanode 1101 and the cathode 1102 is not limited to two. A light-emittingelement shown as an example in FIG. 10C has a structure in which aplurality of light-emitting units 1103 are stacked, that is, a so-calledtandem structure. Note that in the case where n (n is a natural numbergreater than or equal to 2) light-emitting units 1103 are providedbetween the anode and the cathode, the interlayer 1104 is providedbetween an m-th (m is a natural number greater than or equal to 1 andless than or equal to n−1) light-emitting unit and an (m+1)-thlight-emitting unit.

A light-emitting unit 1103 includes at least a light-emitting layercontaining a light-emitting substance, and may have a structure in whichthe light-emitting layer and a layer other than the light-emitting layerare stacked. Examples of the layer other than the light-emitting layerare layers containing a substance having a high hole-injection property,a substance having a high hole-transport property, a substance having apoor hole-transport property (substance which blocks holes), a substancehaving a high electron-transport property; a substance having a highelectron-injection properly, and a substance having a bipolar property(substance having high electron- and hole-transport properties).

An example of a specific structure of the light-emitting unit 1103 isillustrated in FIG. 10B. In the light-emitting unit 1103 illustrated inFIG. 10B, a hole-injection layer 1113, a hole-transport layer 1114, alight-emitting layer 1115, an electron-transport layer 1116, and anelectron-injection layer 1117 are stacked in this order from the anode1101 side.

A specific example of a structure of the interlayer 1104 is illustratedin FIG. 10A. The interlayer 1104 may be formed to include at least acharge generation region, and may have a structure in which the chargegeneration region and a layer other than the charge generation regionare stacked. For example, a structure can be employed in which a firstcharge generation region 1104 c, an electron-relay layer 1104 b, and anelectron-injection buffer layer 1104 a are stacked in this order fromthe cathode 1102 side.

The behaviors of electrons and holes in the interlayer 1104 aredescribed. When a voltage higher than the threshold voltage of thelight-emitting element is applied between the anode 1101 and the cathode1102, in the first charge generation region 1104 c, holes and electronsare generated, and the holes move into the light-emitting unit 1103 b onthe cathode 1102 side and the electrons move into the electron-relaylayer 1104 b.

The electron-relay layer 1104 b has a high electron-transport propertyand immediately transfers the electrons generated in the first chargegeneration region 1104 c to the electron-injection buffer layer 1104 a.The electron-injection buffer layer 1104 a can reduce a barrier againstelectron injection into the light-emitting unit 1103, so that theefficiency of the electron injection into the light-emitting unit 1103is increased. Thus, the electrons generated in the first chargegeneration region 1104 c are injected into the LUMO level of thelight-emitting unit 1103 through the electron-relay layer 1104 b and theelectron-injection buffer layer 1104 a.

In addition, the electron-relay layer 1104 b can prevent interaction inwhich the substance included in the first charge generation region 1104c and the substance included in the electron-injection buffer layer 1104a react with each other at the interface thereof and the functions ofthe first charge generation region 1104 c and the electron-injectionbuffer layer 1104 a are damaged.

The holes injected into the light-emitting unit 1103 b provided on thecathode side are recombined with the electrons injected from the cathode1102, so that a light-emitting substance contained in the light-emittingunit emits light. The electrons injected into the light-emitting unitprovided on the anode side are recombined with the holes injected fromthe anode side, so that a light-emitting substance contained in thelight-emitting unit emits light. Thus, the holes and electrons generatedin the interlayer 1104 cause light emission in the respectivelight-emitting units.

Note that the light-emitting units can be provided in contact with eachother when these light-emitting units allow the same structure as theinterlayer to be formed therebetween. Specifically, when one surface ofthe light-emitting unit is provided with a charge generation region, thecharge generation region functions as a first charge generation regionof the interlayer; thus, the light-emitting units can be provided incontact with each other.

Note that an interlayer can be provided between the cathode and the n-thlight-emitting unit.

Material for Light-Emitting Element

Next, specific materials that can be used for the light-emitting elementhaving the above-described structure are described. Materials for theanode, the cathode, the EL layer, the charge generation region, theelectron-relay layer, and the electron-injection buffer layer aredescribed in this order.

Material for Anode

The anode 1101 is preferably formed using a metal, an alloy, anelectrically conductive compound, a mixture of these materials, or thelike which has a high work function (specifically, a work function ofhigher than or equal to 4.0 eV is more preferable). Specifically, forexample, indium tin oxide (ITO), indium tin oxide containing silicon orsilicon oxide, indium zinc oxide (IZO), indium oxide containing tungstenoxide and zinc oxide, and the like are given.

Besides, as a material used for the anode 1101, the following can begiven: gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium(Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium(Pd), titanium (Ti), nitride of a metal material (e.g., titaniumnitride), molybdenum oxide, vanadium oxide, ruthenium oxide, tungstenoxide, manganese oxide, titanium oxide, and the like.

Note that in the case where a second charge generation region isprovided in contact with the anode 1101, a variety of conductivematerials can be used for the anode 1101 regardless of their workfunctions. Specifically, besides a material which has a high workfunction, a material which has a low work function can also be used forthe anode 1101. A material for forming the second charge generationregion will be subsequently described together with a material forforming the first charge generation region.

Material for Cathode

As a material of the cathode 1102, a material having a low work function(specifically, a work function of lower than 4.0 eV) is preferably used;however, in the case where the first charge generation region isprovided between the cathode 1102 and the light-emitting unit 1103 to bein contact with the cathode 1102, various conductive materials can beused for the cathode 1102 regardless of their work functions.

Note that at least one of the cathode 1102 and the anode 1101 is formedusing a conductive film that transmits visible light. For the conductivefilm that transmits visible light, for example, a film of indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide, indium zinc oxide, and indium tinoxide to which silicon oxide is added can be given. Further, a metalthin film having a thickness small enough to transmit light (preferably,approximately 5 nm to 30 nm) can also be used.

Material for EL Layer

Specific examples of materials for the layers included in thelight-emitting unit 1103 will be given below.

Hole-Injection Layer

The hole-injection layer contains a substance having a highhole-injection property. As the substance having a high hole-injectionproperty, for example, molybdenum oxide, vanadium oxide, rutheniumoxide, tungsten oxide, manganese oxide, or the like can be used. Inaddition, it is possible to use a phthalocyanine-based compound such asphthalocyanine (abbreviation: H₂Pc) or copper phthalocyanine(abbreviation: CuPc), a polymer such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), orthe like to form the hole-injection layer.

Note that the second charge generation region may be used instead of thehole-injection layer. When the second charge generation region is used,a variety of conductive materials can be used for the anode 1101regardless of their work functions as described above. A material forforming the second charge generation region will be subsequentlydescribed together with a material for forming the first chargegeneration region.

Hole-Transport Layer

The hole-transport layer contains a substance having a highhole-transport property. The hole-transport layer is not limited to asingle layer, but may be a stack of two or more layers each containing asubstance having a high hole-transport property. The hole-transportlayer contains any substance having a higher hole-transport propertythan an electron-transport property, and preferably contains a substancehaving a hole mobility of 10⁻⁶ cm²V·s or higher because the drivingvoltage of the light-emitting element can be reduced.

Light-Emitting Layer

The light-emitting layer contains a light-emitting substance. Thelight-emitting layer is not limited to a single layer, but may be astack of two or more layers each containing a light-emitting substance.As the light-emitting substance, a fluorescent compound or aphosphorescent compound can be used. A phosphorescent compound ispreferably used as the light-emitting substance because the emissionefficiency of the light-emitting element can be increased.

The light-emitting substance is preferably dispersed in a host material.A host material preferably has higher excitation energy than thelight-emitting substance.

Electron-Transport Layer

The electron-transport layer contains a substance having a highelectron-transport property. The electron-transport layer is not limitedto a single layer, but may be a stack of two or more layers eachcontaining a substance having a high electron-transport property. Theelectron-transport layer contains any substance having a higherelectron-transport property than a hole-transport property, andpreferably contains a substance having an electron mobility of 10⁻⁶cm²/V·s or higher because the driving voltage of the light-emittingelement can be reduced.

Electron-Injection Layer

The electron-injection layer contains a substance having a highelectron-injection property. The electron-injection layer is not limitedto a single layer, but may be a stack of two or more layers eachcontaining a substance having a high electron-injection property. Theelectron-injection layer is preferably provided because the efficiencyof electron injection from the cathode 1102 can be increased and thedriving voltage of the light-emitting element can be reduced.

As the substance having a high electron-injection property, thefollowing can be given: an alkali metal and an alkaline earth metal suchas lithium (Li), cesium (Cs), calcium (Ca) and a compound thereof, suchas lithium fluoride (LiF), cesium fluoride (CsF), and calcium fluoride(CaF₂). Alternatively, a layer containing a substance having anelectron-transport property and an alkali metal, an alkaline earthmetal, magnesium (Mg), or a compound thereof (e.g., an Alq layercontaining magnesium (Mg)) can be used.

Material for Charge Generation Region

The first charge generation region 1104 c and the second chargegeneration region are regions containing a substance having a highhole-transport property and an acceptor substance. Note that the chargegeneration region is not limited to the structure in which one filmcontains the substance having a high hole-transport property and theacceptor substance, and may be a stacked layer of a layer containing thesubstance having a high hole-transport property and a layer containingthe acceptor substance. Note that in the case where the first chargegeneration region which is in contact with the cathode has astacked-layer structure, the layer containing the substance having ahigh hole-transport property is in contact with the cathode 1102. In thecase where the second charge generation region which is in contact withthe anode has a stacked-layer structure, the layer containing theacceptor substance is in contact with the anode 1101.

Note that the acceptor substance is preferably added to the chargegeneration region so that the mass ratio of the acceptor substance tothe substance having a high hole-transport property is from 0.1:1 to4.0:1.

As the acceptor substance that is used for the charge generation region,a transition metal oxide, particularly an oxide of a metal belonging toGroup 4 to 8 of the periodic table is preferable. Specifically,molybdenum oxide is particularly preferable. Note that molybdenum oxidehas a low hygroscopic property.

As the substance having a high hole-transport property used for thecharge generation region, any of a variety of organic compounds such asan aromatic amine compound, a carbazole derivative, an aromatichydrocarbon, and a high molecular compound (including an oligomer, adendrimer, or a polymer) can be used. Specifically, a substance having ahole mobility of 10⁻⁶ cm²/V·s or higher is preferably used. However, anysubstance other than the above described materials may also be used aslong as the substance has a higher hole-transport property than anelectron-transport property.

Material for Electron-Relay Layer

The electron-relay layer 1104 b can immediately receive electrons drawnout by the acceptor substance in the first charge generation region 1104c. Therefore, the electron-relay layer 1104 b contains a substancehaving a high electron-transport property, and the LUMO level thereof ispositioned between the acceptor level of the acceptor substance in thefirst charge generation region 1104 c and the LUMO level of thelight-emitting unit 1103. Specifically, the LUMO level of theelectron-relay layer 1104 b is preferably about from −5.0 eV to 3.0 eV.

As the substance used for the electron-relay layer 1104 b, for example,a perylene derivative and a nitrogen-containing condensed aromaticcompound can be given. Note that a nitrogen-containing condensedaromatic compound is preferably used for the electron-relay layer 1104 bbecause of its stability. Among nitrogen-containing condensed aromaticcompounds, a compound having an electron-withdrawing group such as acyano group or fluorine is preferably used because such a compoundfurther facilitates acceptance of electrons in the electron-relay layer1104 b.

Material for Electron-Injection Buffer Layer

The electron-injection buffer layer 1104 a facilitates electroninjection from the first charge generation region 1104 c into thelight-emitting unit 1103. By providing the electron-injection bufferlayer 1104 a between the first charge generation region 1104 c and thelight-emitting unit 1103, the injection barrier therebetween can bereduced.

A substance having a high electron-injection property can be used forthe electron-injection buffer layer 1104 a. For example, an alkalimetal, an alkaline earth metal, a rare earth metal, or a compoundthereof (e.g., an alkali metal compound (including an oxide such aslithium oxide, a halide, and carbonate such as lithium carbonate orcesium carbonate), an alkaline earth metal compound (including an oxide,a halide, and carbonate), or a rare earth metal compound (including anoxide, a halide, and carbonate)) can be used.

Further, in the case where the electron-injection buffer layer 1104 acontains a substance having a high electron-transport property and adonor substance with respect to the substance having a highelectron-transport property, the donor substance is preferably added sothat the mass ratio of the donor substance to the substance having ahigh electron-transport property is from 0.001:1 to 0.1:1. Note that asthe donor substance, an organic compound such as tetrathianaphthacene(abbreviation: TTN), nickelocene, or decamethylnickelocene can be usedas well as an alkali metal, an alkaline earth metal, a rare earth metal,a compound of the above metal (e.g., an alkali metal compound (includingan oxide such as lithium oxide, a halide, and carbonate such as lithiumcarbonate or cesium carbonate), an alkaline earth metal compound(including an oxide, a halide, and carbonate), and a rare earth metalcompound (including an oxide, a halide, and carbonate). Note that as thesubstance having a high electron-transport property, a material similarto the above material for the electron-transport layer which can beformed in part of the light-emitting unit 1103 can be used.

Method for Manufacturing Light-Emitting Element

A method for manufacturing the light-emitting element will be described.Over the first electrode, the layers described above are combined asappropriate to form an EL layer. Any of a variety of methods (e.g., adry process or a wet process) can be used for the EL layer depending onthe material for the EL layer. For example, a vacuum evaporation method,an inkjet method, a spin coating method, or the like may be selected.Note that a different method may be employed for each layer. The secondelectrode is formed over the EL layer, so that the light-emittingelement is manufactured.

The light-emitting element described in this embodiment can befabricated by combination of the above-described materials. Lightemission from the above-described light-emitting substance can beobtained with this light-emitting element, and the emission color can beselected by changing the type of the light-emitting substance.

Further, a plurality of light-emitting substances which emit light ofdifferent colors can be used, whereby, for example, white light emissioncan also be obtained by expanding the width of the emission spectrum. Inorder to obtain white light emission, for example, a structure may beemployed in which at least two layers containing light-emittingsubstances are provided so that light of complementary colors isemitted. Specific examples of complementary colors include “blue andyellow”, “blue-green and red”, and the like.

Further, in order to obtain white light emission with an excellent colorrendering property, an emission spectrum preferably spreads through theentire visible light region. For example, a light-emitting element mayinclude layers emitting light of blue, green, and red.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 5

In this embodiment, electronic devices according to embodiments of thepresent invention will be described. Specifically, electronic devices oneach of which the display device shown as an example any of Embodiments1 to 4 is mounted will be described with reference to FIGS. 11A to 11E.

Examples of such an electronic device for which a display deviceaccording to one embodiment of the present invention is used include thefollowing: television sets (also called TV or television receivers);monitors for computers or the like; cameras such as digital cameras ordigital video cameras; digital photo frames; mobile phones (also calledcellular phones or portable telephones); portable game machines;portable information terminals; audio playback devices; and large gamemachines such as pachinko machines. Specific examples of theseelectronic devices are shown in FIGS. 11A to 11E.

FIG. 11A shows an example of a television set. In a television set 7100,a display portion 7103 is incorporated in a housing 7101. Images can bedisplayed on the display portion 7103. In addition, here, the housing7101 is supported by a stand 7105.

The television set 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television set 7100 is provided with a receiver, a modem,and the like. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (between a sender and a receiver orbetween receivers) information communication can be performed.

FIG. 11B shows a computer that includes a main body 7201, a housing7202, a display portion 7203, a keyboard 7204, an external connectionport 7205, a pointing device 7206, and the like. The display device ofone embodiment of the present invention is used for the display portion7203 in this computer.

FIG. 11C shows a portable game machine that includes two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.A display portion 7304 is incorporated in the housing 7301 and a displayportion 7305 is incorporated in the housing 7302. In addition, theportable game machine illustrated in FIG. 11C includes a speaker portion7306, a recording medium insertion portion 7307, an LED lamp 7308, aninput means (an operation key 7309, a connection terminal 7310, a sensor7311 (a sensor having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredrays), or a microphone 7312), and the like. It is needless to say thatthe structure of the portable game machine is not limited to the aboveas long as the display device of one embodiment of the present inventionis used for at least either the display portion 7304 or the displayportion 7305, or both, and can include other accessories arbitrarily.The portable game machine illustrated in FIG. 11C has a function ofreading a program or data stored in a recording medium to display it onthe display portion, and a function of sharing information with anotherportable game machine by wireless communication. The portable gamemachine in FIG. 11C can have a variety of functions without limitationto the above functions.

FIG. 11D shows an example of a mobile phone. A mobile phone 7400 isprovided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400is manufactured by using the display device of one embodiment of thepresent invention for the display portion 7402.

When the display portion 7402 is touched with a finger or the like, datacan be input into the mobile phone 7400 in FIG. 11D. Further, operationssuch as making a call and creating e-mail can be performed by touch onthe display portion 7402 with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or creating e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be input. In this case, itis preferable to display a keyboard or number buttons on almost all thearea of the screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 7400, display on the screen of the display portion 7402 canbe automatically changed by determining the orientation of the mobilephone 7400 (whether the mobile phone is placed horizontally orvertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on kinds of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed within a specified period while a signal detectedby an optical sensor in the display portion 7402 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

FIG. 11E shows an example of a folding computer. A folding computer 7450includes a housing 7451L and a housing 7451R connected by hinges 7454.The folding computer 7450 further includes an operation button 7453, aleft speaker 7455L, and a right speaker 7455R. In addition, a sidesurface of the folding computer 7450 is provided with an externalconnection port 7456, which is not illustrated. When the hinge 7454 isfolded so that a display portion 7452L provided in the housing 7451L anda display portion 7452R provided in the housing 7451R face each other,the display portions can be protected by the housings.

Each of the display portions 7452L and 7452R is a component which candisplay images and to which information can be input by touch with afinger or the like. For example, an icon for an installed program isselected by touch with a finger so that the program can be started.Further, changing the distance between fingers touching two positions ofa displayed image enables zooming in or out on the image. Drag of afinger touching one position of the displayed image enables drag anddrop of the image. Selection of a displayed character or symbol on thedisplayed image of a keyboard by touch with a finger enables informationinput.

Further, the computer 7450 can also include a gyroscope, an accelerationsensor, a global positioning system (GPS) receiver, fingerprint sensor,or a video camera. For example, a detection device including a sensorwhich detects inclination, such as a gyroscope or an accelerationsensor, is provided to determine the orientation of the computer 7450(whether the computer is placed horizontally or vertically for alandscape mode or a portrait mode) so that the orientation of thedisplay screen can be automatically changed.

Furthermore, the computer 7450 can be connected to a network. Thecomputer 7450 not only can display information on the Internet but alsocan be used as a terminal which controls another electronic deviceconnected to the network from a distant place. The display device of oneembodiment of the present invention is used for the display portion7452L and the display portion 7452R in the folding computer 7450.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

This application is based on Japanese Patent Application Serial No.2012-107944 filed with Japan Patent Office on May 9, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method of driving a semiconductor device,comprising: generating a signal that corrects a response time of lightemission of a light-emitting module and outputting the signal to adisplay portion, so that a response time of light emission of thelight-emitting module is longer than or equal to 1 μs and shorter than 1ms, wherein the semiconductor device comprises the display portion inwhich pixels are provided at a resolution of greater than or equal to 80ppi, wherein the light-emitting module capable of emitting light with aspectral line half-width of less than or equal to 60 nm is in each pixelof the pixels, wherein the semiconductor device has an NTSC ratio ofhigher than or equal to 80% and a contrast ratio of higher than or equalto
 500. 2. A method of driving a semiconductor device, comprising:generating a signal that corrects a response time of light emission of alight-emitting module and outputting the signal to a display portion, sothat a response time of light emission of the light-emitting module islonger than or equal to 1 μs and shorter than 1 ms, wherein thesemiconductor device comprises the display portion in which pixels areprovided at a resolution of greater than or equal to 80 ppi, wherein thelight-emitting module capable of emitting light with a spectral linehalf-width of less than or equal to 60 nm is in each pixel of thepixels, wherein the semiconductor device has an NTSC ratio of higherthan or equal to 80% and a contrast ratio of higher than or equal to500, wherein the signal comprises a first signal with a voltage which islower than a voltage corresponding to a desired luminance and a secondsignal with a voltage corresponding to the desired luminance.